1. Field of the Invention
This invention concerns a technique for the output of fonts on high resolution output devices.
2. Prior Art
High resolution output devices include, for example, electronic phototypesetters or high resolution laser printers. These devices typically include a separate processor unit, which receives the text as such to be output, for example from an external computer, and with the application of digitally stored fonts converts it into a pre-set font form of particular font type and size. A margin adjustment can also be taken into consideration. Data output by the processor unit contains complete information about the font image and is fed into an interim storage, which serves to activate an output unit which transfers the data, for example with the help of a laser beam, onto a duplicating medium such as a film or paper.
In a typical computer-controlled phototypesetter technique, digitally stored fonts are provided, whereby outer line coding has caught on extensively, in which the contours or edges of a letter are described through discrete control points and connected curve elements, as displayed in FIG. 1. For example the X-, Y-co-ordinates of the start, corner, curve and tangent points are digitized as points and the curve elements are described by the various manufacturers as straight and circle, as Bezier functions, as spirals or as Spline functions. An overview of the digital font formats is to be found in the book "Digital Fonts", Peter Karow, Springer-Verlag, Berlin Heidelberg, 1992 (which is hereby incorporated by reference). Aside from the control points the letters receive so-called instructions for the definition of the outer line, which are used with the rastering for the output of the letter, for example on a laser printer or a screen, and are today above all applied by all manufacturers of digital fonts for "intelligent rastering" (Intelligent Font Scaling) (see the book "Font Technology", Peter Karow, Springer-Verlag, Berlin Heidelberg, 1992; Chapter 7 which is hereby incorporated by reference).
With exacting text designs exists the need to vary the fonts, saved in their design size, according to application purpose and design desires in the font image. Thus one may generate font size (point size) and width (alphabet length) as well as to create the desired weight. With digitally stored fonts of today, most often a master font for each font type is used and this is re-scaled according to the desired display. This technique is, however, not sufficient to obtain a typographically correct result.
Font sizes are measured in a typographical unit, which is labelled as point (pt), and it is: 1 pt=0.375 mm. The linear re-scaling of digitally stored fonts generally used today is unsatisfactory in various aspects. The font sizes manufactured through linear enlargement/reduction of a master are at the moment in their legibility and the total aesthetic impression clearly inferior to optically correct and typographically appropriate manufactured font sizes. The typographically correct "Optical Scaling", as was applied previously with the special manufacture of the different font sizes of a font type, takes into consideration that the resolution capacity of the human eye is limited. With small printed text (e.g. 5 pt) we need more space between the letters than with normal book text (e.g. 9 pt), in order that the letters do not run together and the small text remains readable. The same applies for small white counters such as in the small letter e. Very thin strokes (e.g. swash lines, so-called hair lines) need to be strengthened by the small font sizes, in order that they remain at all recognizable and don't "break through", i.e. partly disappear, already during the printing process. Conversely, if one proceeds to bigger inscriptions (advertising, placards, titles) the letters can be placed relatively closely together throughout, white counters can remain small and hair lines thin. This allows, if one starts from a master in the design size of the font, a summary in approximately the following way.
The smaller the font size,
1) therefore the wider the sentence, PA1 2) therefore the more open the letters, and PA1 3) therefore the thicker the stroke flow PA1 1) therefore the narrower the sentence, PA1 2) therefore the letters can be thinner and PA1 3) therefore particularly the hair lines can be finer. PA1 Optical scaling to the desired point size PA1 Expansion/condensation to the desired alphabet length, and PA1 Generation of the desired weight,
The bigger the font size,
In the redated application EP 92250199.4, a technique for the output of fonts is proposed with which, starting from a single master font, optical scaling is automatically practicable. With the technique described there, a master font stored in a middle point size is first linearly re-scaled with a factor to the desired point size and subsequently the instructions for it are applied to steadily vary the stroke width of the letters, linearly re-scaled by the factor, in a primary pre-set factor dependency, whereby in order to achieve an optical scaling the stroke width is increased for factors smaller than 1% and is reduced for factors larger than 1%.
The problem of expansion/condensation plays a role for the typographically correct sentence. Previously this requirement could only be satisfied through the offer of expanded or respectively condensed fonts. These broader, or respectively narrower variations of a print type had to be produced through expensive hand work. Therefore for cost reasons they were not manufactured and made available for most fonts. The simple linear broadening/narrowing of letters, as is customary in many cases today in the computer-controlled phototypesetting technique, does not lead to typographically correct expanded/condensed fonts since thereby the letters are broadened or respectively narrowed in their entirety i.e. both the black letter strokes as well as the white counters in the letters and between the letters of the typeset word. For typographically correct expanded/condensed fonts the white space may be altered but not the stroke widths. In the related application EP 92250199.4 the applicant proposes a technique for the output of fonts in which, starting from a single master font, the fonts to be output are brought to the desired width through linear expansion or condensation with a factor, and subsequently the instructions for it are applied in order to hold the stroke widths constant during the linear expansion or condensation of the letters, and in this way to only broaden or narrow the counters.
In March 1991 the firm Adobe Systems, California, presented a concept called "Multiple Master", in which for the first time automatically a font display with:
can be manufactured automatically in a single system.
The combination of the three font varying operations (optical scaling, expansion/condensation, generation of a weight) is schematically displayed in FIG. 2. The font interpolation is displayed by means of a dice, whereby along the first axis the point size (optical scaling), along the second axis the alphabet length (expansion/condensation) and along the third axis the weight is steadily varied. The representation of a desired font is based on linear interpolation between stored master fonts with different point sizes, alphabet lengths, and weights. Consequently eight master fonts are necessary in order to be able to create a font display with the desired point size, alphabet length and weight. These eight master fonts correspond to the eight corner points of the dice displayed in FIG. 2. A grave disadvantage of this technique lies in the fact that eight raster fonts are needed, which has on the one hand as a result a high storage space requirement and on the other hand means a large time expenditure for the manufacture of the master fonts. Aside from this, the following difficulties also arise: with the manufacture of eight master fonts it is necessary to simultaneously keep the eight variations of a letter under control, i.e. all eight variants must be provided with digitized data such that they remain able to be linearly interpolated. Particular attention must be paid that in all cases the digitizing points are equal in number, type and position and that the instructions for the "Intelligent Font Scaling" are equal in number, type and parameterization. If later, for example, an improvable error in the appearance (digitizing points) or with the processing (instructions) occurs in only one letter, one must similarly correct the seven other pertinent letter variations, which in total leads to considerable time expenditure in the manufacture of the master fonts.